Cardiac rhythm management system with defibrillation threshold prediction

ABSTRACT

A cardiac rhythm management device predicts defibrillation thresholds without any need to apply defibrillation shocks or subjecting the patient to fibrillation. Intravascular defibrillation electrodes are implanted in a heart. By applying a small test energy, an electric field near one of the defibrillation electrodes is determined by measuring a voltage at a sensing electrode offset from the defibrillation electrode by a known distance. A desired minimum value of electric field at the heart periphery is established. A distance between a defibrillation electrodes and the heart periphery is measured, either fluoroscopically or by measuring a voltage at an electrode at or near the heart periphery. Using the measured electric field and the measured distance to the periphery of the heart, the defibrillation energy needed to obtain the desired electric field at the heart periphery is estimated. In an example, the device also includes a defibrillation shock circuit and a stimulation circuit.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.10/744,991, filed on Dec. 23, 2003, which is a continuation of U.S.patent application Ser. No. 09/808,419, filed on Mar. 14, 2001, nowissued as U.S. Pat. No. 6,751,502. This application also claims priorityto U.S. patent application Ser. No. 10/921,777, filed Aug. 18, 2004, andto Bocek et al. U.S. Provisional Patent Application Ser. 60/600,614,filed Aug. 11, 2004, entitled PACEMAKER WITH COMBINED DEFIBRILLATORTAILORED FOR BRADYCARDIA PATIENTS. The above patent and patentapplications are incorporated by reference in their entirety.

TECHNICAL FIELD

The present system relates generally to cardiac rhythm managementsystems and particularly, but not by way of limitation, to a systemproviding, among other things, defibrillation threshold prediction.

BACKGROUND

When functioning properly, the human heart maintains its own intrinsicrhythm, and is capable of pumping adequate blood throughout the body'scirculatory system. However, some people have irregular cardiac rhythms,referred to as cardiac arrhythmias. Such arrhythmias result indiminished blood circulation. One mode of treating cardiac arrhythmiasuses drug therapy. Drugs are often effective at restoring normal heartrhythms. However, drug therapy is not always effective for treatingarrhythmias of certain patients. For such patients, an alternative modeof treatment is needed. One such alternative mode of treatment includesthe use of a cardiac rhythm management system. Such systems are oftenimplanted in the patient and deliver therapy to the heart.

Cardiac rhythm management systems include, among other things,pacemakers, also referred to as pacers. Pacers deliver timed sequencesof low energy electrical stimuli, called pace pulses, to the heart, suchas via an intravascular leadwire or catheter (referred to as a “lead”)having one or more electrodes disposed in or about the heart. Heartcontractions are initiated in response to such pace pulses (this isreferred to as “capturing” the heart). By properly timing the deliveryof pace pulses, the heart can be induced to contract in proper rhythm,greatly improving its efficiency as a pump. Pacers are often used totreat patients with bradyarrhythmias, that is, hearts that beat tooslowly, or irregularly.

Cardiac rhythm management systems also include defibrillators that arecapable of delivering higher energy electrical stimuli to the heart.Such defibrillators also include cardioverters, which synchronize thedelivery of such stimuli to portions of sensed intrinsic heart activitysignals. Defibrillators are often used to treat patients withtachyarrhythmias, that is, hearts that beat too quickly. Such too-fastheart rhythms also cause diminished blood circulation because the heartisn't allowed sufficient time to fill with blood before contracting toexpel the blood. Such pumping by the heart is inefficient. Adefibrillator is capable of delivering an high energy electricalstimulus that is sometimes referred to as a defibrillation countershock,also referred to simply as a “shock.” The countershock interrupts thetachyarrhythmia, allowing the heart to reestablish a normal rhythm forthe efficient pumping of blood. In addition to pacers, cardiac rhythmmanagement systems also include, among other things,pacer/defibrillators that combine the functions of pacers anddefibrillators, drug delivery devices, and any other implantable orexternal systems or devices for diagnosing or treating cardiacarrhythmias.

One problem faced by cardiac rhythm management systems is thedetermination of the threshold energy required, for a particulardefibrillation shock waveform, to reliably convert a tachyarrhythmiainto a normal heart rhythm. Ventricular and atrial fibrillation areprobabilistic phenomena that observe a dose-response relationship withrespect to shock strength. The ventricular defibrillation threshold(VDFT) is the smallest amount of energy that can be delivered to theheart to reliably revert ventricular fibrillation to a normal rhythm.Similarly, the atrial defibrillation threshold (ADFT) is the thresholdamount of energy that will terminate an atrial fibrillation. Suchdefibrillation thresholds vary from patient to patient, and may evenvary within a patient depending on the placement of the electrodes usedto deliver the therapy. In order to ensure the efficacy of such therapyand to maximize the longevity of the battery source of such therapyenergy, the defibrillation thresholds must be determined so that thedefibrillation energy can be safely set above the threshold value butnot at so large of a value so as to waste energy and shorten the usablelife of the implanted device.

One technique for determining the defibrillation threshold is to inducethe targeted tachyarrhythmia (e.g., ventricular fibrillation), and thenapply shocks of varying magnitude to determine the energy needed toconvert the arrhythmia into a normal heart rhythm. However, thisrequires imposing the risks and discomfort associated with both thearrhythmia and the therapy. Electrical energy delivered to the heart hasthe potential to both cause myocardial injury and subject the patient topain. Moreover, if defibrillation thresholds are being obtained in orderto assist the physician in determining optimal lead placement, thesedisadvantages are compounded as the procedure is repeated for differentpotential lead placements.

Another technique for determining the defibrillation threshold, referredto as the “upper limit of vulnerability” technique, a patient in a stateof normal heart rhythm is shocked during the vulnerable (T-wave) periodof the cardiac cycle during which time the heart tissue is undergoingrepolarization. Shocks of varying magnitude are applied untilfibrillation is induced. Of course, after such fibrillation is induced,the patient must be again shocked in order to interrupt the arrhythmiaand reestablish a normal heart rhythm. In this technique, thecorresponding fibrillation-inducing shock magnitude is then related to adefibrillation threshold energy using a theoretical model. The upperlimit of vulnerability technique also suffers from imposing the risksand discomfort associated with both the arrhythmia and the shocktherapy. Moreover, because of the discomfort associated with thefibrillation and countershocks, the patient is typically sedated undergeneral anesthesia, which itself has some additional risk and increasedhealth care cost. For these and other reasons, there is a need toestimate defibrillation thresholds without relying on a defibrillationshock to induce or terminate an actual arrhythmia.

SUMMARY

The present system provides, among other things, a cardiac rhythmmanagement system that predicts defibrillation thresholds without anyneed to apply defibrillation shocks or subjecting the patient tofibrillation. In one embodiment, the system provides a method thatincludes delivering a nondefibrillating and nonfibrillation-inducingtest energy to a heart, detecting a resulting output signal based on thetest energy and a heart characteristic, and estimating a defibrillationthreshold, based on the output signal, for a portion of the heart to bedefibrillated. In an embodiment, a method also includes detecting atleast one intrinsic electrical heart signal from a heart of a patientand delivering to the heart a stimulation at an energy level appropriateto evoke or assist in evoking a responsive heart contraction. In anembodiment, the method also includes detecting a ventriculartachyarrhythmia or fibrillation using a technique having a specificityand a sensitivity, wherein the specificity exceeds the sensitivity, anddelivering a shock in response to at least the detected tachyarrhythmiaor fibrillation, the shock in excess of the threshold voltage associatedwith the second electric field strength.

In one embodiment, the system includes first and second electrodesconfigured for association with a heart. A test energy module is coupledto the second electrode, for delivering a nondefibrillating andnonfibrillation-inducing test energy to the heart. A response signalmodule is coupled to the first and second electrodes for detectingresponses to the test energy. A controller is coupled to the responsesignal module. The controller estimates a defibrillation thresholdenergy based on a predetermined desired defibrillation electric field ata distal portion of the heart tissue to be defibrillated and a distancefrom the second electrode to the distal portion of the heart tissue, andan indication of the electric field near the second electrode. Otheraspects of the invention will be apparent on reading the followingdetailed description of the invention and viewing the drawings that forma part thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsdescribe substantially similar components throughout the several views.Like numerals having different letter suffixes represent differentinstances of substantially similar components. The drawings illustrategenerally, by way of example, but not by way of limitation, variousembodiments discussed in the present document.

FIG. 1 is a schematic/block diagram illustrating portions of the presentcardiac rhythm management system and portions of an environment of use.

FIG. 2 is a flow chart illustrating a technique for estimatingdefibrillation thresholds such as using the system of FIG. 1.

FIG. 3 is a lookup table illustrating estimating defibrillationthreshold voltages based on an indication of electric field near adefibrillation electrode and a distance therefrom.

FIG. 4 is a schematic/block diagram illustrating an alternativeembodiment of portions of the present cardiac rhythm management systemthat determines a distance from the defibrillation electrode withoutrequiring fluoroscopic or other imaging.

FIG. 5 is a flow chart, similar to that of FIG. 2, illustrating anothermethod of operation such as using the system of FIG. 4.

FIG. 6 is a flow chart and accompanying graph illustrating one techniquefor estimating a distance from an electrode.

FIG. 7 is a flow chart illustrating another embodiment providing anindicator of the predicted defibrillation threshold and/or selecting anappropriate cardiac rhythm management device for implant.

FIG. 8 is a flow chart illustrating another embodiment providing adefibrillation shock based on the predicted defibrillation thresholdenergy.

FIG. 9 is a flow chart illustrating another embodiment for recordingacute or chronic computed defibrillation thresholds.

FIG. 10 is a flow chart illustrating another embodiment for modifyingthe delivery of a defibrillation shock or other therapy based onpreviously acquired defibrillation threshold data over a range ofanother patient characteristic.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims and their equivalents. In thedrawings, like numerals describe substantially similar componentsthroughout the several views. Like numerals having different lettersuffixes represent different instances of substantially similarcomponents. The term “and/or” refers to a nonexclusive “or” (i.e., “Aand/or B” includes both “A and B” as well as “A or B”).

The present methods and apparatus will be described in applicationsinvolving implantable medical devices including, but not limited to,implantable cardiac rhythm management systems such as pacemakers,cardioverter/defibrillators, pacer/defibrillators, biventricular orother multi-site coordination devices, and drug delivery systems formanaging cardiac rhythm. However, it is understood that the presentmethods and apparatus may be employed in unimplanted devices, including,but not limited to, external pacemakers, cardioverter/defibrillators,pacer/defibrillators, biventricular or other multi-site coordinationdevices, monitors, programmers and recorders.

EXAMPLE A

FIG. 1 is a schematic/block diagram illustrating generally, by way ofexample, and not by way of limitation, one embodiment of portions of thepresent cardiac rhythm management system 100 and portions of anenvironment in which the present system 100 and associated techniquesare used. System 100 includes, among other things, cardiac rhythmmanagement device 105 and leadwire (“lead”) 110 for communicatingsignals between device 105 and a portion of a living organism, such asheart 115. Embodiments of device 105 include, but are not limited to,bradycardia and antitachycardia pacemakers, cardioverters,defibrillators, combination pacemaker/defibrillators, drug deliverydevices, and any other implantable or external cardiac rhythm managementapparatus capable of providing therapy to heart 115.

In this example, lead 110 includes multiple electrodes, and individualconductors for independently communicating an electrical signal fromeach electrode to device 105. In one embodiment, these electrodesinclude a right ventricular (RV) tip-type electrode 120 at the distalend of lead 110. In one example embodiment, electrode 120 has amacroscopic surface area that is approximately between 1 mm² and 20 mm²,inclusive. RV tip electrode 120 is configured to be positioned in theright ventricle at or near its apex or at any other suitable location.RV shock electrode 125 is located on the lead at a known predetermineddistance, d1, from RV tip electrode 120, as measured from the edges ofthese electrodes. RV shock electrode 125 is typically located in theright ventricle or at any other suitable location. In one embodiment, RVshock electrode 125 is a coil-type electrode having a macroscopicsurface area that is approximately between 2 cm² and 20 cm², inclusive.Superior vena cava (SVC) electrode 130 is located in a portion of thesuperior vena cava, the right atrium, or both, or at any other suitablelocation. In one embodiment, SVC electrode 130 is a coil-type electrodehaving a macroscopic surface area that is approximately between 2 cm²and 20 cm², inclusive. Although RV tip electrode 120, RV shock electrode125, and SVC electrode 130 are particularly described above with respectto structural characteristics and locations for disposition, it isunderstood that these electrodes may take the form of any of the variouscardiac electrodes known in the art (e.g., epicardial patch electrodes)and may be positioned elsewhere for association with heart 115 or othertissue.

In one embodiment, device 105 includes a hermetically sealed housing135, formed from a conductive metal, such as titanium, and implantedwithin a patient such as within the pectoral or abdominal regions. Inthis example, housing 135 (also referred to as a “case” or “can”) issubstantially covered over its entire surface by a suitable insulator,such as silicone rubber, except for at a window that forms a “case” or“can” or “housing” electrode 140. As understood by one of ordinary skillin the art, housing electrode 140, although not located in the heart, isassociated with the heart for providing what is sometimes referred to as“unipolar” sensing or pacing or defibrillation therapy. In oneembodiment, a header 145 is mounted on housing 135, such as forreceiving lead 110. Header 145 is formed of an insulative material, suchas molded plastic. Header 145 also includes at least one receptacle,such as for receiving lead 110 and electrically coupling conductors oflead 110 to device 105. Header 145 may also include one or moreadditional electrodes. In this example, ventricular fibrillation istreated by delivering a defibrillation shock between RV shock electrode125 and the commonly connected combination of SVC electrode 130 andhousing electrode 140; a defibrillation threshold is also obtained withSVC electrode 130 and housing electrode 140 connected in common.However, it is understood that these electrodes could be differentlyconfigured, such as for delivering defibrillation therapy between RVshock electrode 125 and housing electrode 140.

FIG. 1 also illustrates, in an exploded view block diagram form,portions of device 105. It is understood that device 105 is coupled toheart 115 via lead 110; the illustrated connection lines associated withthe exploded view are illustrative only. In FIG. 1, test energy module150 generates an energy from which a heart characteristic can bedetermined via response signal module 155. From these measurements, adefibrillation threshold is computed, for example, either by adefibrillation threshold estimation module in controller 160, whichitself is in device 105, or in external programmer 170, which iscommunicatively coupled to a transmitter or receiver in device 105, suchas transceiver 175. The defibrillation estimation module is implementedas a sequence of steps carried out on a microprocessor or othermicrosequencer, in analog, digital, or mixed-signal hardware, or in anysuitable hardware and/or software configuration. In this example, SVCelectrode 130 is electrically connected in common with housing electrode140, at node 165, and also coupled to each of test energy module 150 andresponse signal module 155. The device 105 also includes a heart signalsensing module 156, a defibrillation shock module 157, and a stimulationmodule 158. In an example, the test energy module 150 includes at leasta portion of the shock module 157 or the stimulation module 158. Inanother example, the response signal module 155 includes at least aportion of the heart signal sensing module 156.

FIG. 2 is a flow chart illustrating generally, by way of example, butnot by way of limitation, one embodiment of a technique for estimatingdefibrillation thresholds such as using the system 100 of FIG. 1. Thistechnique is carried out as executable instructions, such as bycontroller 160, but it need not be carried out in the exact sequenceillustrated in FIG. 2. At step 200, test energy module 150 applies atest energy by providing a drive current of predetermined magnitude(e.g., approximately 30 to 1000 microamperes, inclusive) between RVshock electrode 125 and housing electrode 140. In one example, thisdrive current is delivered in a continuous or pulsed/strobed 25 kHzwaveform; in this example the 30 to 1000 microamperes current magnitudesare the zero-to-peak values of this test waveform. However, it isunderstood that the technique could use any other test energy that doesnot defibrillate the associated heart tissue and does not inducefibrillation, such as when the energy is delivered during a cardiacrepolarization or by using a non-painful stimulus such as a pacing pulse(e.g., amplitude approximately between 0.1 Volt and 10 Volts, inclusive,duration approximately between 0.05 milliseconds and 10 milliseconds,inclusive). In one embodiment, the test energy typically has an energyless than 10 milliJoules, while typical defibrillation thresholdenergies are between 1 and 40 Joules. The test energy may be deliveredfrom either a current source or a voltage source.

At step 205, response signal module 155 measures a response voltage V1between RV shock electrode 125 and RV tip electrode 120. At step 210,response signal module 155 also measures a response voltage V2 betweenRV tip electrode 120 and housing electrode 140. One such embodiment ofproviding a test current and sensing a resulting voltage is described inHartley et al. U.S. Pat. No. 6,076,015 (“the Hartley et al. patent”)entitled “RATE ADAPTIVE CARDIAC RHYTHM MANAGEMENT DEVICE USINGTRANSTHORACIC IMPEDANCE,” assigned to Cardiac Pacemakers, Inc., thedisclosure of which is incorporated herein by reference in its entirety,including its incorporation by reference of Hauck et al., U.S. Pat.5,318,597 entitled “RATE ADAPTIVE CARDIAC RHYTHM MANAGEMENT DEVICECONTROL ALGORITHM USING TRANS-THORACIC VENTILATION, also assigned toCardiac Pacemakers, Inc. Although the Hartley et al. patent is directedtoward providing a test current and sensing a resulting voltage tomeasure transthoracic impedance, those same techniques and structuresfor carrying out such techniques, including the use of a continuous orpulsed/strobed high frequency carrier signal (e.g., at a frequency thatis approximately between 1 kHz and 100 kHz, inclusive), are alsoapplicable here. Such techniques are employed either using the electrodeconfiguration illustrated in FIG. 1, or using the electrodeconfiguration described or incorporated by reference in the Hartley etal patent, or by using any other suitable electrode configuration thatdisposes electrodes for association with heart 115 for providing a testsignal and/or detecting a response signal.

In this example, at step 215, a fractional tip voltage parameterV2/(V1+V2) is determined by controller 160 or external programmer 170.This fractional tip voltage parameter provides a measured indication ofthe electric field distribution near RV shock electrode 125, because itrelates to a voltage drop over a known distance d1.

In this example, at step 220, the attending physician or other usermeasures a distance d2 from the RV shock electrode the outer peripheryof the left ventricular apex. In one embodiment, this distance ismeasured by viewing an image of the heart on a fluoroscope or otherimaging apparatus, using the known distance d1 to assess the distanced2. Based on the distance d2 and the fractional tip voltage parameter, adefibrillation threshold is estimated, at step 225, using a model ofelectric field distribution (for example, having foci at RV shockelectrode 125 and at SVC electrode 130, as illustrated in this exampleby the elliptical lines superimposed on heart 115 in FIG. 1) thatprovides a desired electric field magnitude throughout the heart,including its periphery.

In one embodiment, a generalized electric field distribution for theparticular lead electrode configuration is calculated a priori using theknown lead electrode geometry and the boundary element method, orsimilar method, to solve the electric field Laplace equation. In oneexample, commercially available finite element analysis software is usedto solve the electric field distribution for the particular electrodearrangement. A resulting generalized equation describing the electricfield is then obtained using a cubic fit. For a particular electrodeconfiguration, the measured voltage at tip electrode 120 may bedifferent from that obtained from the generalized electric fieldequation for the modeled electrode geometry, for example, because of asmall variation in the distance between tip electrode 120 and coilelectrode 125, and/or because of the particular tissue and/or bloodconductivity characteristics of the patient. To obtain additionalaccuracy, these small variations are accounted for by scaling orotherwise calibrating the generalized electric field equation such thatit obtains substantially the same electric field at the tip electrode120 as the measured electric field at tip electrode 120, an indicationof which is given by the fractional tip voltage parameter. Using theresulting calibrated electric field equation and measured distance tothe heart periphery, the electric field obtained at the heart peripheryin response to the test voltage is calculated. The defibrillationthreshold voltage, then, is calculated by scaling the test voltagedelivered at coil electrode 125 by the ratio of the desired electricfield at the heart periphery needed for successful defibrillation (e.g.,5 Volts/cm) to the extrapolated value of electric field at the heartperiphery obtained from the calibrated electric field equation inresponse to the test voltage stimulus.

In this example, it has been assumed that a minimum electric fieldmagnitude of 5 Volts/cm at the left ventricular periphery of heart 115(at a distance d2 away from RV shocking electrode 125) is required toconvert ventricular fibrillation into a normal heart rhythm (e.g., witha 50% probability). A more conservative user might select a largerpredetermined desired electric field intensity (e.g., 6 V/cm) at theheart periphery. Moreover, other experimental data might indicate that alower electric field intensity (e.g., 4 V/cm) is sufficient to obtain asuccessful defibrillation. It is understood that the present system andtechniques are applicable and may be used in conjunction with anydesired electric field intensity at the distal point (from thedefibrillation electrode) of the tissue being defibrillated.

For ease of use, such as in an implantable device, the defibrillationthreshold voltage is, in one embodiment, stored in a lookup table in amemory device. FIG. 3 illustrates generally, by way of example, but notby way of limitation, one embodiment of such a lookup table. In thisembodiment, the defibrillation threshold voltage needed to obtain a 5Volts/cm electric field at the left ventricular heart periphery is givenas a function of: (1) the measured distance d2 less the known distanced1; and (2) the fractional tip voltage parameter. Thus, the lookup tablein FIG. 3 represents solving the electric field distribution for aparticular lead geometry and a range of various heart sizes, calibratingthe resulting cubic-fitted electric field equations according todifferent measured values of electric field as indicated by the range offractional tip voltage parameters, and obtaining the correspondingdefibrillation threshold voltage by scaling the test voltage by theratio of the “safe” value of electric field at the heart periphery tothe extrapolated value of the electric field at the heart periphery asobtained from the calibrated electric field equation in response to thetest voltage delivered from the defibrillation coil electrode 120. Thus,FIG. 3 indicates, for example, for a measured distance, (d2−d1), of 3.4cm and a fractional tip voltage parameter of 45.0, then the predicteddefibrillation threshold voltage given by the table in FIG. 3 is 408Volts.

In this way, the detected electric field (represented by the fractionaltip voltage parameter) associated with the detected voltage between RVshock electrode 125 and SVC electrode 130 in response to the test energyis scaled upward to ensure an adequate minimum electric field at theheart periphery (and, therefore, an adequate electric field throughoutthat portion of the heart tissue being defibrillated). The associatedscaled voltage that provides the desired electric field at the heartperiphery is deemed the appropriate defibrillation threshold voltage.

Stated differently, in summary, for a given defibrillation waveform, thecorresponding defibrillation threshold voltage is obtained as follows.An elliptical or other (e.g., spherical, quadratic, exponential,polynomial, or other approximation of electric field) electric fieldmodel is used to extrapolate the electric field at a distal portion ofthe heart tissue to be defibrillated (e.g., the left ventricularperiphery in this example) based on the electric field measured near thedefibrillation electrode (e.g., the RV shock electrode 125 in thisexample) and the measured distance d2 to the distal portion of the hearttissue to be defibrillated. The defibrillation threshold is obtained byscaling the measured voltage at the defibrillation electrode by theratio of the desired defibrillation electric field at the distal portionof the heart to be defibrillated to the test value of electric field atthat distal portion as obtained by the previous measurement and modeledextrapolation. For this particular example, then, the estimateddefibrillation threshold voltage, VDFT, is represented byVDFT=V₁₂₅*(E_(LV,DESIRED)/E_(LV, MODEL)); in this equation, V₁₂₅ is thevoltage measured (or calculated) at RV shock electrode 125 in responseto the test current, E_(LV,DESIRED) is the desired electric field at theleft ventricular periphery for proper defibrillation (in this case,assumed to be 5 V/cm), and E_(LV, MODEL) is the electric field at theleft ventricular periphery based on the electric field measurement nearRV shock electrode 125 and the extrapolation over the distance d2 usingthe elliptical or other electric field model.

It should be understood that the defibrillation threshold voltage couldfurther be scaled upward to provide a safety margin of additionaldefibrillation energy. Moreover, although this example describedmeasuring d2 from the RV shocking electrode 125 to the left ventricularperiphery, it is understood that the measurement d2 may be performedbetween any electrode used to deliver defibrillation energy to anyportion of heart 115 and a distal portion of the heart tissue that isfarthest from the defibrillation electrode but for which an adequatedefibrillation electric field is desired.

EXAMPLE B

FIG. 4 is a schematic/block diagram illustrating generally, by way ofexample, and not by way of limitation, another embodiment of portions ofsystem 100 providing an alternate embodiment of determining the distanced2, such as described with respect to step 220 of FIG. 2, that does notrequire the use of fluoroscopic or other imaging. FIG. 4 includes anadditional peripheral electrode 400 located at or close to theperipheral portion of the left ventricle (a distance d2 away from RVshock electrode 125) at which the predetermined electric field magnitude(e.g., 5 Volts/cm, as in the previous example) is desired duringdefibrillation. In one embodiment, this peripheral electrode 400 isintroduced into the left ventricular periphery (e.g., coronary sinusand/or great cardiac vein) by an transvascular lead 405 through theright atrium and coronary sinus. In another embodiment, peripheralelectrode 400 is a patch-type defibrillation electrode disposed on theexterior portion of the left ventricle. In either case, lead 405 mayalso include additional electrodes.

FIG. 5 is a flow chart, similar to that of FIG. 2, illustratinggenerally, by way of example and not by way of limitation, anothermethod of operation such as using the embodiment illustrated in FIG. 4.This technique is carried out by executable instructions, such as oncontroller 160, but it need not be carried out in the exact sequenceindicated in FIG. 5. At step 500, an additional voltage measurement V3is taken between peripheral electrode 400 and housing electrode 140 inresponse to the current delivered by test energy module 150 at step 200.At step 505 the distance d2 from RV shock electrode 125 to the heartperiphery electrode 400 is estimated without relying on fluoroscopic orother imaging techniques to make a measurement. Instead, the distance d2is estimated using the measured voltage V3 obtained in step 500.

Referring again to FIG. 5, at 226, at least one intrinsic electricalheart signal is detected from a heart of a patient. In one example, thisis a ventricular signal that, at least during normal ventricularrhythms, includes QRS complexes indicative of ventriculardepolarizations. Such ventricular signals also include discernablecharacteristics indicative of ventricular tachyarrhythmias, such as aventricular fibrillation or polymorphic ventricular tachyarrhythmia(PVT) episode to be treated by an electrical shock to the heart. At 227,one or more stimulations are delivered to the heart, if needed to treata bradyarrhythmia or as part of a cardiac resynchronization therapy(CRT) that is intended to improve spatial coordination of the heartcontraction to improve cardiac output. Any such stimulations aredelivered at an energy level (e.g., at a pacing-type energy level) thatis appropriate to evoke or assist in evoking a responsive heartcontraction. At 228, a determination is made of whether a shockablearrhythmia is detected. Examples of a shockable arrhythmia includeventricular fibrillation (VF) or a shockable polymorphic ventriculartachycardia. This detection is performed using a technique having aspecificity and a sensitivity, such as from a particular combination ofparameters used in detecting the shockable arrhythmia and indelivering/inhibiting shock therapy. In one example, the specificityexceeds the sensitivity. In one example, the determination of whether ashockable arrhythmia exists includes (or, alternatively, consists of)determining whether a ventricular heart rate exceeds a high ratethreshold value, such as 220 beats per minute. At 228, if a shockablearrhythmia is detected then, at 229, a shock is delivered in response tothe VT/VF, either alone or in combination with one or more othertriggers. The shock is intended to terminate the VT/VF such that theheart reverts back to a non-tachyarrhythmic rhythm. Process flow thenreturns to 226. At 228, if a shockable arrhythmia is not detected, thenprocess flow returns to 226.

FIG. 6 is a flow chart and accompanying graph illustrating generally, byway of example, but not by way of limitation, one technique forestimating the distance d2, at step 505. In this technique, the electricfield near RV shock electrode 125 is approximated, as a function ofdistance, as a decaying exponential, for distances measured radiallyoutward from RV shock electrode 125. By using (V1+V2) and V2 as pointson this exponential curve that are separated by the known distance d1,as illustrated in FIG. 6, a decay rate “R” (i.e., the argument of thedecaying exponential function) is computed at step 600. Then, at step605, the distance d2 is estimated using the previously determined decayrate R. Having determined the distance d2 without relying onfluoroscopic imaging techniques, the defibrillation threshold energy isestimated as previously described herein with respect to FIGS. 1-3, orby other suitable technique.

OTHER EXAMPLES

FIG. 7 is a flow chart illustrating generally, by way of example, butnot by way of limitation, another embodiment of using system 100. Thisembodiment includes steps for estimating defibrillation thresholdvoltages for a particular defibrillation waveform delivered from aparticular electrode configuration, such as described with respect toFIG. 5 (or FIG. 2). Then, at step 700, an indication of thedefibrillation threshold energy is provided to the user. In one example,the defibrillation threshold energy estimated within device 105 iscommunicated by telemetry transceiver 175 to external programmer 170 fordisplay, such as on a computer monitor, audible output, printed means,or using any other indicator. In another example, the defibrillationthreshold energy is estimated by hardware included within externalprogrammer 170, which is itself coupled to lead 110 with or withoutactually implanting a device 105. A resulting indication of thedefibrillation threshold energy is displayed on programmer 170. Based onthis indicated defibrillation threshold energy, the user selects anappropriate cardiac rhythm management device 105 for implantation. Inthis way, an implantable cardiac rhythm management device 105 having alarger battery capacity is selected for a patient exhibiting a largerdefibrillation threshold voltage than for a patient exhibiting a lesserdefibrillation threshold voltage. This selection of a cardiac rhythmmanagement device 105 having appropriate energy capacity may also bebased on other factors, including, by way of example, but not by way oflimitation, the expected frequency of needed defibrillation episodes,the patient's need for other power-consuming features in the implantablecardiac rhythm management device. Thus, according to this technique ofcomputing defibrillation thresholds for a particular electrodeconfiguration, the user may advantageously determine the appropriatecardiac rhythm management device 105 before actually performing animplantation.

FIG. 8 is a flow chart illustrating generally, by way of example, butnot by way of limitation, another embodiment of using system 100. Thisembodiment includes steps for estimating defibrillation thresholdvoltages for a particular defibrillation waveform and providing anindicator of the defibrillation threshold energy to the user, asdescribed with respect to FIG. 7. Then, at step 800, a defibrillationshock having a magnitude based on the predicted defibrillation thresholdenergy (e.g., equal to the predicted defibrillation threshold energy) isdelivered to a patient in fibrillation to test whether the applieddefibrillation shock magnitude is sufficient to defibrillate thepatient. If the defibrillation is successful, the user may again testefficacy using a lesser defibrillation shock; if the defibrillation isnot successful, the user may again test efficacy using a greaterdefibrillation shock.

FIG. 9 is a flow chart illustrating generally, by way of example, butnot by way of limitation, another embodiment of using system 100. Thisembodiment includes steps for estimating defibrillation thresholdvoltages for a particular defibrillation waveform (as described withrespect to FIGS. 2 and 5). At step 900, the computed defibrillationthreshold energy and corresponding time is stored in memory incontroller 160. After a delay at step 905, the defibrillation thresholdestimation steps are repeated and the resulting defibrillation thresholdenergy and time are again recorded and stored. The stored defibrillationthreshold energy and corresponding time data is, in one embodiment,communicated to external programmer 170 by transceiver 175. In oneembodiment, a relatively short delay (e.g., approximately between 1 hourand 1 day, inclusive) is used at step 905, during a period of timeimmediately following implantation of defibrillation lead 110. In thisway, acute changes in defibrillation threshold are monitored and stored.In another. embodiment, a longer delay (e.g., approximately between 1day and 1 month, inclusive) is used at step 905. In this way, chronicchanges in defibrillation threshold are monitored and stored. Suchchronic changes in defibrillation threshold provide one indication ofpatient well-being and suitability for continued use of the cardiacrhythm management system 100.

FIG. 10 is a graph of transthoracic impedance (Z) versus time. FIG. 10illustrates another aspect of the present system 100 in which a patientcharacteristic, such as breathing (also referred to as respiration orventilation) is monitored. One technique for monitoring breathing is bymeasuring transthoracic impedance, as described in Hartley et al. U.S.Pat. No. 6,076,015 entitled “RATE ADAPTIVE CARDIAC RHYTHM MANAGEMENTDEVICE USING TRANSTHORACIC IMPEDANCE,” assigned to Cardiac Pacemakers,Inc., which is incorporated herein by reference in its entirety. Asillustrated in FIG. 10, defibrillation thresholds are repeatedlycomputed, according to the techniques described herein, at severaldifferent times during the patient's breathing cycle of inhaling andexhaling. An indication of the portion of the breathing cycle thatcorresponds to the lowest computed defibrillation threshold is recorded.In one example, this is implemented by recording a transthoracicimpedance corresponding to the lowest defibrillation threshold. Inanother embodiment, this is implemented by recording a time delay from afiducial point of the thoracic impedance waveform (e.g., maxima, minima,“zero”-crossing, etc.). Then, at some later time when the patient is infibrillation, a defibrillation shock is delivered by system 100synchronized to (among other things) the portion of the respirationcycle that was found to correspond to the lowest defibrillationthreshold energy. In a broader sense, because the defibrillationthreshold estimation techniques disclosed herein do not require anactual defibrillation energy or fibrillation-inducing energy, suchdefibrillation threshold estimation can be carried out repeatedly forevaluation over a range of any other patient characteristics (e.g.,posture, etc.) besides breathing. Variations in the defibrillationthreshold energy may then be used to synchronize delivery of thedefibrillation shock to that particular patient characteristic, or tootherwise modify therapy delivery.

CONCLUSION

The above-discussed system provides, among other things, an apparatusand methods for estimating defibrillation thresholds without having toinduce an arrhythmia or provide a defibrillation shock, and therebyavoids the disadvantages associated therewith, as discussed above.Although the system has been so described to illustrate this one of itsadvantages, it is not limited in this way. Stated differently, thesystem could also be used in conjunction with techniques that inducearrhythmias and/or deliver defibrillation countershocks to determinedefibrillation thresholds.

The above-discussed system has been particularly described in terms ofits use to determine ventricular defibrillation thresholds. However, itis understood that the described technique could also be used todetermine atrial or other defibrillation thresholds by simplyrepositioning the electrodes to be associated with the atrial tissue tobe defibrillated. Moreover, the described system need not be confined toa use in determining defibrillation thresholds; it could also be usedfor determining the required applied voltage at any electrode that isneeded to obtain a desired electric field at a distance away therefrom.

The systems and methods described herein for painlessly estimating adefibrillation threshold will be particularly useful in a cardiac rhythmmanagement device having both bradyarrhythmia pacing therapy anddefibrillation shock therapy capabilities, where the device isconfigured to be used in a patient population that is not normallyindicated for an antitachyarrythmia therapy device. In such patients,erroneously or unnecessarily delivering a defibrillation shock isparticularly undesirable, as explained in the above-incorporated U.S.patent application Ser. No. 10/921,777. However, includingdefibrillation capability in such devices is still useful for preventingpatient mortality. Therefore, such devices in such a patient populationare particularly well-suited for the present systems and methods ofpainlessly estimating a defibrillation threshold, since the presentsystems and methods avoid any need to actually deliver a defibrillationshock during defibrillation threshold testing.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments may be used in combination with each other. Many otherembodiments will be apparent to those of skill in the art upon reviewingthe above description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled. In the appendedclaims, the term “including” is used as being equivalent to the term“comprising,” and its interpretation should not be limited to“physically including” unless otherwise indicated.

1. A system for evaluating a heart, the system including: a test energymodule adapted to deliver a test energy to the heart; a response signalmodule adapted to detect responding signals providing for an indicationof an electric field strength in a predetermined cardiac region, theresponding signals resulted from the delivery of the test energy; and acontroller in communication with the test energy module and the responsesignal module, the controller adapted to estimate a defibrillationthreshold based on the indication of the electric field strength.
 2. Thesystem of claim 1, further including a voltage source from which thetest energy is delivered.
 3. The system of claim 1, further including acurrent source from which the test energy is delivered.
 4. The system ofclaim 1, further including a first lead having a distal end configuredto be positioned in the heart, the first lead including a plurality ofelectrodes to deliver the test energy and detect one or more of theresponding signals.
 5. The system of claim 4, in which the first leadincludes: a first electrode, coupled to the response signal module andlocated at the distal end of the lead, to detect at least one of theresponding signals; and a second electrode, coupled to the test energymodule and the response signal module and located at a predetermineddistance from the first electrode, to detect at least one of theresponding signals and deliver the test energy.
 6. The system of claim5, in which the second electrode is a defibrillation electrode.
 7. Thesystem of claim 6, in which the first lead includes a third electrode,coupled to the test energy module and the response signal module, todetect at least one of the responding signals and deliver the testenergy.
 8. The system of claim 7, further including a second leadincluding a fourth electrode to detect a voltage signal indicative of adistance between the first electrode and the fourth electrode.
 9. Anapparatus comprising: a test energy circuit adapted to deliver a testenergy to the heart; a response signal circuit adapted to detectresponding signals providing for an indication of an electric fieldstrength in a predetermined cardiac region, the responding signalsresulted from the delivery of the test energy; a controller incommunication with the test energy circuit and the response signalcircuit, the controller adapted to estimate a defibrillation thresholdbased on the indication of the electric field strength; a heart signalsensing circuit to sense intrinsic electrical heart signals from a heartof a patient; a ventricular tachyarrhythmia/fibrillation detectorcircuit, operatively coupled to the heart signal sensing circuit, theventricular tachyarrhythmia/fibrillation detector circuit operable todetect a ventricular tachyarrhythmia/fibrillation, wherein theventricular tachyarrhythmia/fibrillation detector circuit has asensitivity and a specificity, and wherein the ventriculartachyarrhythmia/fibrillation detector circuit is configured such thatthe specificity exceeds the sensitivity; a defibrillation shock circuit,coupled to the ventricular tachyarrhythmia/fibrillation detectorcircuit, the defibrillation shock circuit configured to deliver adefibrillation shock in response to the detected ventriculartachyarrhythmia/fibrillation, the defibrillation shock exceeding thedefibrillation threshold; and a stimulation circuit, coupled to theheart signal sensing circuit, the stimulation circuit configured todeliver to the heart a stimulation at an energy level appropriate toevoke or assist in evoking a responsive heart contraction.
 10. Theapparatus of claim 9, wherein the test energy circuit includes at leasta portion of the defibrillation shock circuit.
 11. The apparatus ofclaim 9, wherein the test energy circuit includes at least a portion ofthe stimulation circuit
 12. The apparatus of claim 9, wherein theresponse signal circuit includes at least a portion of the heart signalsensing circuit.
 13. The apparatus of claim 9, in which the specificityis greater than or equal to 99%.
 14. The apparatus of claim 13, in whichthe sensitivity is less than 99%.
 15. The apparatus of claim 13, inwhich the specificity is greater than or equal to 99.5%.
 16. Theapparatus of claim 15, in which the sensitivity is less than 99.5%. 17.The apparatus of claim 9, in which the ventriculartachyarrhythmia/fibrillation detector circuit includes one or more shockcontrol modules to determine whether a patient should be shocked,wherein the one or more shock control modules are individually orcollectively programmable by one or more parameters, and wherein the oneor more parameters are factory programmed to one or more correspondingdefault values such that the specificity exceeds the sensitivity in atarget patient population.
 18. A method for estimating a thresholdvoltage for defibrillating a heart, the method including: delivering atest energy to the heart; determining an indication of a first electricfield strength near a first cardiac location from which the test energyis delivered, the first electric field strength resulting from thedelivery of the test energy; estimating a second electric fieldstrength, near a second cardiac location, using the indication of thefirst electric field strength, the second electric field strengthassociated with a threshold voltage, the second electric field strengthresulting from the delivery of the test energy; detecting at least oneintrinsic electrical heart signal from a heart of a patient; deliveringto the heart a stimulation at an energy level appropriate to evoke orassist in evoking a responsive heart contraction; detecting aventricular tachyarrhythmia or fibrillation using a technique having aspecificity and a sensitivity, wherein the specificity exceeds thesensitivity; and delivering a shock in response to at least the detectedtachyarrhythmia or fibrillation, the shock in excess of the thresholdvoltage associated with the second electric field strength.
 19. Themethod of claim 18, in which delivering the test energy includesdelivering a test signal having a test voltage, the method furtherincluding calculating the threshold voltage by scaling the test voltageby a ratio of a predetermined minimum electric field strength to theestimated second electric field strength.
 20. The method of claim 18,further comprising factory programming one or more shock controlparameters such that the specificity exceeds the sensitivity in a targetpatient population.